Research On Swing Arm Profilometer Test For Large-aperture Complex Optical Surface

Optics with large aperture provide high angular resolution and powerful energy gathering ability.Optics with complex aspheric surface offer more parameters for optical system design and acquire less optical elements.Large-aperture complex aspherics are wildly used in astronomical observation and earth observation system and so on.As the one of the most important part of the telescopes,the large-aperture complex aspherics are developed with much funds and physical resources.The general fabrication process of large optical surfaces includes generating,grinding and polishing.For those complex optical surfaces,in order to meet the requirements of all stages,the metrologies are difficult,especially for grinding process.During generating stage the shape was guided by the measuring probe on the generating machine itself.For polishing stage,the surface was tested by interferometer.For grinding stage,the accuracy of the complex surface should be covering from 0.1mm to 0.001 mm level,which means the range of the testing should cover 102 order of magnitudes.With the advantage of high efficient and in situ test and not limited by the large size of optics,instead of traditional Coordinate measuring machine(CMM),swing arm profilometer(SAP)was used for profile measurement of large or extremely large aspherics.This paper presents a detailed research on technology of the SAP to achieve a high accurate and high efficient test.The main work of this paper are as follows:(1)Analyzing misalignment errors of SAP.For improving the test accuracy,the important guarantee is supposed to be accurate alignment of each component.As one of most significant error sources that affects the test accuracy,misalignment error leads to low order errors such as aspherical aberrations,coma etc.apart from power.Based on the structure of SAP system,the model containing each alignment parameters was build.As relation between alignment parameters and test result of axi-symmetric optics is presented,the analytical solutions of SAP system errors from tested mirror misalignment,arm length L deviation,tilt angle ? deviation,air-table spin error and air-table misalignment are derived respectively.To verify the analytical model,an experiment on a 2-m diameter parabolic mirror was demonstrated.So the errors induced from misalignment for testing any complex aspherics with its surface parameters given,can be calculated exactly.This helps to offer the misalignment budget quickly and precisely and offer a theoretical guides for alignment.(2)System errors analysis of SAP.Effect on surface test from each component of SAP was discussed,and then the displacement sensor and the air-table was calibrated.To analyze test errors induced from system noise of swing arm profilometer(SAP)testing large off-axis aspherics with different scan arcs,the simulation test model was built based on the Monte Carlo method.The measurement sensibility for different surface residual in low order terms and high order terms.(3)High precision SAP test equipped with a capacitive displacement sensor.As RB-Si C mirror is electric conductive,a method was developed for improving the measurement accuracy of SAP for testing its surface by using a capacitive probe.To ensure the accuracy of the probe,calibration on an ultra-precise single-point diamond turning(SPDT)machine was implemented,the dynamic stability about temperature and the linearity ofcapacitive probe was tested,and error induced from the tilt factor between target and probe was discussed.Experimental test with SAP was performed to measure a 1000 × 800 mm RB-Si C mirror to verify the measurement ability.The accuracy of the surface measurement was 0.06?m RMS.(4)Stitching SAP test with multi-probes in dual position.To improve the test accuracy of SAP,multi-probes were used to achieve on-line and real-time calibrating for system errors.The mode of the multi-probes were designed and analyzed.Two probes were put side by side along the scan arcs to calibrate the vibration of air-table;and the third probe was put under the air-table to record the noise of SAP system during the measurement.The calibration algorithm was discussed in detail.For extremely large aperture complex aspherics,instead of a longer arm amplifying the system errors,a short arm stitching test in two different position was proposed.Taking 4-m diameter optics as an example,the noise sensitivity of different stitching mode was simulated,and the optimizing mode of two position was selected.Taking sampling density and consuming time into account,an additional probe was set to increase the sampling density without increase the numbers of the scan lines,and the relative position of the probe was solved given the sampling density in edge of the surface.(5)SAP test large aspheric optics.First,a 1.6m plane optic was tested for several iterations of grinding.Then a 1.5m off-axis aspheric mirror was tested from initial grinding phase to initial polishing phase.The surface from SAP test matched well with the Interferometer test result.Finally,a 2-m axi-symmetric parabolic mirror was tested.Guided by SAP test,the 2-m mirror was grinded and polished to surface residual of 1.6?m PV and 0.2?m RMS.Compared with Interferometer test,the test accuracy of SAP was 0.86?m PV and 0.17?m RMS.